VAX 4000-500 PSU Overload?

Robert Jarratt robert.jarratt at
Wed Dec 23 09:48:44 CST 2015

> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at] On Behalf Of Brent
> Hilpert
> Sent: 21 December 2015 22:33
> To: General Discussion: On-Topic and Off-Topic Posts
> Subject: Re: VAX 4000-500 PSU Overload?
> SMD markings and packagings are ambiguous, unclear, and all over the
> spectrum.
> Here's a good site for chasing SMD markings:
> REing boards like this is an iterative process with consideration of the
> markings, pin use, measurements, what makes sense electronically, and
> design period.
> In the two cases you mention it looks like the H's are a slightly
different font
> or size and spaced slightly differently, I suspect the id codes there are
A4 and
> 2F.
> An initial guess for the "A4H" device that's right above the LM339 is an
> code dual diode with only one diode used, as it looks (from the photo)
> one pin is unconnected.
> A good guess for the "2FH" devices are 2F-code PNP transistors:
> Similarly, the "1PG" or  "1PC" device may be a 1P-code NPN transistor.
> Diodes and transistors are best double-checked by first characterising
> multimeter with PN junctions in known transistors or diodes so you know
> what ohms-range and what reading to expect on your MM when you have a
> PN junction in forward-conduction. Look for the lowest ohms range that
> give you a reading for forward conduction. Then use that knowledge to
> the SMD transistors and diodes for sensibility.
> Some strategy:
> 	- Sort out all the network connections for the board, by visual
> observation and continuity measurement.
> 	- Identify the power supply pins to the board by chasing the power
> supply pins of the IC packages.
> 	  This will include or begin with identifying the ground
> to the board in part by chasing the
> 	  ground from the main unit to the pins which this board plugs into.
> 	  Is an internal layer on the board a ground plane?
> 	-  From what I can discern it may be that the two ICs are supplied
> +/- supplies, that is, the V- pin for the
> 	  ICs may go to a negative supply rather than ground, so there may
> ground(s), V- and V+ connections
> 	  to the board, rather than just ground and V+.
> 	- I'd suggest drawing the two op amps one above the other with a
> positive bus horizontally at the top of the page.
> 	  If there is a negative supply draw a negative bus horizontally at
> bottom of the page.
> 	  Draw in the networks around the op amps, it looks like they are
> similar and feed into the 3 terminal device located to the upper-right of
> LM339,
> 	  and thence into one of the comparators. That should take care of a
> lot of the board, then do the remaining 3 comparators.
> 	- within sensibility, try to draw everything so electron flow is
'up' the
> page, this means for example that PNP transistors
> 	 will be drawn upside down, with the emitter at top-right, while
> will be conventional with emitter at lower-right.
> 	 Everything should sit vertically in between the negative bus below
> and positive bus above or between grounds towards the bottom and the
> positive bus above.
> 	- within sensibility, try to keep signal flow left-to-right with
> feedback paths going 'backwards' right-to-left.

I reckon I have got about as far as I can with reverse engineering the riser
board that connects to the current sensing resistors. I am less certain now
that the fault could be in this area, but I thought that I might as well
finish the schematic since I have got so far with it. The result is here:
The part under the "Riser Board" label is as complete as I can make it, the
part under the "Output Stage" label is just the very last part of the board
that the riser is connected to. The X3-L and X3-R connectors are the
connectors on the riser board. There is much more on the board the riser is
attached to,  mainly transformers, large resistors and capacitors, plus
another little riser board with a UC3825 PWM controller on it; it also has
tracks in a middle layer, I can see them a bit, but they are very hard to

There are likely some mistakes. For example, not having the datasheets for
what are probably transistors and dual diodes, I may have got them the wrong
way round, although I am pretty sure the pin that is on its own on one side
of the package is the common cathode for the dual diodes, and the base for
the transistors. I am not sure how useful the schematic is going to be. It
would be good if I could test it in isolation on the bench, but it looks
like it would be a difficult set up, and I am not at all certain that the
fault is in this area anyway.

I think the best thing now would be if I could test the whole +12V/-12V
sub-assembly on the bench. It is fed somewhat indirectly from other parts of
the PSU, but it isn't clear to me if the input is DC or AC, although I
suspect AC. I have uploaded two annotated pictures as well at the same URL
(note that the 12V board has a number of missing parts at the moment). I
suspect the best thing is probably going to be to add some probe wires at
the 12V board input and measure them with the PSU installed (so it detects
the fan and doesn't shutdown), to see if power is even getting to it.



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